Hypothesis / aims of study
The urinary tract is involved in 24% of congenital anomalies, which can cause voiding difficulties, infections, or renal impairment. Posterior urethral valves (PUV) are one of those malformations in boys, with an incidence of 1:3800-1:8000 live male births. PUV can result in severe bladder outflow obstruction (BOO), resulting in higher detrusor pressures during voiding, increasing the risk of voiding difficulties, urinary tract infections, or renal impairment.
The gold standard for assessing the existence of BOO is a urodynamic study (UDS). During the voiding phase of a UDS, the ratio between the detrusor pressure (pdet) and the urine flowrate (Q) is assessed. In adults, several quantification methods for BOO were proposed, which were found comparable in their discrimination between BOO and no BOO. Although well-defined in the adult population, no method for the assessment of urethral resistance in children is defined. The International Children Continence Society (ICCS) standard only reports on average higher voiding pressures in children, while no comment is made about measures for urethral obstruction.
An additional complicating factor in determining the existence of BOO in young children is their inability to sit on a uroflowmetry toilet, resulting in the fact that the urine flowrate is not available. As the urethral resistance is by definition a ratio of pressure and flowrate, the urethral resistance cannot be assessed in these children, and the presence of BOO cannot be determined.
Although a direct urine flowrate measurement using standard equipment is not possible in young children, we hypothesize that the flow can be derived from other modalities. In children, UDS is often supplemented with X-ray images to assess the morphology of the lower urinary tract, called video urodynamic study (VUDS). In this study, we describe the accuracy of a new method for the flowrate assessment using VUDS.
Study design, materials and methods
25 VUDS’s from children who were able to sit on a uroflowmetry toilet were retrospectively included in this study. During voiding, a sagittal x-ray image was taken at least once per second, as per local protocol. The bladder area on the resulting images was manually segmented. Based on the assumption that the changes in the non-visible third dimension are proportional to changes in the visible dimensions, the volume of the bladder on each successive image was calculated, based on the manually segmented bladder area. The resulting volumes were calibrated using the voided volume and post void residual (if applicable), as this corresponds to the calculated volume of the bladder, just before voiding.
The difference in bladder volume between the successive images was calculated and used as the calculated flow, called ‘videoflow’. This videoflow was compared with the real uroflowmetry from the UDS. The cross-correlation as a general Pearson’s correlation was assessed, and the maximal flow rates (Qmax) according to both methods were compared. Linear regression analysis was used to optimize the calculated Qmax, to make it more accurate and reliable.
Results
Nine boys and 16 girls were included, aged 6-15 years (mean 9.5), resulting in 603 X-ray images included. The comparison between the real uroflowmetry and the videoflow yielded an excellent mean cross-correlation of 0.95 (95% confidence interval: 0.91-0.99). The Qmax was overestimated with on average 3.3ml/s (23%). Stepwise linear regression analysis resulted in a significant linear regression of the Qmax overestimation with the maximal bladder volume (p < 0.001). Age and videoflow-Qmax were not found to have a significant (p > 0.05) correlation with the Qmax overestimation in this regression. After correcting the calculated videoflow-Qmax with the expected error according to the linear regression, the overestimation was reduced to an average difference of 0.04ml/s, (standard deviation 2.0), and the videoflow-Qmax was comparable with the real Qmax (Wilcoxon Signed Rank Test p=0.716).
Interpretation of results
We showed an excellent cross-correlation of the calculated videoflow with the real uroflowmetry during the voiding phase of UDS in children. After correction with linear regression, the videoflow-Qmax was also found accurate. The same method may be used in very young children in whom flowmetry currently cannot be performed. With the help of the calculated videoflow and videoflow-Qmax, the determination of BOO will be possible for the first time in these young children. Demonstration or exclusion of BOO is very important for further clinical management. Nowadays, in case BOO cannot be ruled out, cystoscopy under general anesthesia is performed. A reliable calculation of BOO based on the videoflow may spare young children this invasive procedure. Furthermore, the derivation of the flow pattern besides Qmax (or a more easily accessible average flow, Qavg) is essential. As the use of X-ray is common practice during UDS, no extra radiation is required to calculate the videoflow. The next steps will involve the automatization of the segmentation process, and the inclusion of additional calibration factors, not only based on the full bladder volume but also on multiple moments during filling. In this way, we expect the algorithm to be even more robust and reliable in the calculation of the videoflow.
Concluding message
This study showed that calculation of urinary flow rate based on X-ray images of the bladder in voiding phase of a UDS was accurate in both the estimation of the flowrate curve shape and, after correction, also for the determination of the Qmax. Videoflow may substitute the real uroflowmetry measurement in very young children who are unable to sit on a uroflowmetry toilet. This is important, because with this technique, for the first time, pressure flow study can be performed to determine BOO in very young children.